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Stabilization of Zeolite Particles on Microporous Support Membranes with Spin Coating Method for Thin Film Nanocomposite Membranes

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Even though access to purified water has improved, there are still many people and locations that do not have this convenience. Approximately 1.2 billion people lack access to safe drinking

Even though access to purified water has improved, there are still many people and locations that do not have this convenience. Approximately 1.2 billion people lack access to safe drinking water and 2.6 billion people have little or no sanitation. Furthermore, breakthroughs in water purification technology are essential to combat these issues. While there are several approaches to water purification, membrane processes are widely used based on their numerous advantages, including high operating temperature and low energy input. In essence, membranes do not require chemical additives, thermal inputs, or regeneration of spent media. The spin coating procedure was used to make a total of 94 membrane samples by adjusting the following variables: membrane support, membrane wetting, solvent, polyacrylonitrile (PAN) content, water contant, Linde Type A (LTA) zeolite content, and the rotations per minute (RPM) of the spin coater. Parameters that were held constant include PAN for the permeable dispersion layer, LTA zeolites as the inorganic filler material, and a spin time of 30 seconds for the spin coater. There were key findings in both the preliminary and core data sets. From the preliminary membrane samples 1 \u2014 40, a baseline was established to use for the core data: polysulfone (PSf) support, 1 \u2014 3% PAN content, and 1 \u2014 3% LTA zeolite content. Flux analysis revealed many inconsistencies in groups 1 \u2014 13 such as unreasonably high error bars (+50%), flow rates that were near zero or extremely high (+15,000 L hr-1 m-2), and lack of a clear trend for membrane specifications. Membranes with a high degree of polymer \u2014 zeolite aggregation on the surface had very low flux values. A higher flux of 4,700 L hr-1 m-2 was correlated to gap and hole formation on the membrane surface. It was shown in group 7 that an increasing degree of surface defects corresponded to an increasing flux of 17,000 L hr-1 m-2. Although the target flux for a defect \u2014 free membrane lies between 500 \u2014 4,000 L hr-1 m-2, there were not any groups with flux values in this range. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) analysis revealed that the observed group similarities could not be attributed to individual membrane specifications. However, this data showed chemical fingerprint overlap across all groups, which were synthesized with varying quantities of the same chemicals. Analysis of flux data, SEM images, and ATR-FTIR data all suggest that the spin coating procedure leads to inconsistent results. Although the spin coater provides flexibility in user control, its advantages are outweighed by the limited control of surface uniformity, zeolite dispersion, and defect formation. It has been shown that the spin coating process is not compatible with the formation of a uniform polymer \u2014 zeolite layer in these experiments.

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Date Created
  • 2016-12

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Investigation of Water Permeation through Molecular Sieve Particles in Thin Film Nanocomposite Membranes

Description

Nanoporous materials, with pore sizes less than one nanometer, have been incorporated as filler materials into state-of-the-art polyamide-based thin-film composite membranes to create thin-film nanocomposite (TFN) membranes for reverse osmosis

Nanoporous materials, with pore sizes less than one nanometer, have been incorporated as filler materials into state-of-the-art polyamide-based thin-film composite membranes to create thin-film nanocomposite (TFN) membranes for reverse osmosis (RO) desalination. However, these TFN membranes have inconsistent changes in desalination performance as a result of filler incorporation. The nano-sized filler’s transport role for enhancing water permeability is unknown: specifically, there is debate around the individual transport contributions of the polymer, nanoporous particle, and polymer/particle interface. Limited studies exist on the pressure-driven water transport mechanism through nanoporous single-crystal nanoparticles. An understanding of the nanoporous particles water transport role in TFN membranes will provide a better physical insight on the improvement of desalination membranes.

This dissertation investigates water permeation through single-crystal molecular sieve zeolite A particles in TFN membranes in four steps. First, the meta-analysis of nanoporous materials (e.g., zeolites, MOFs, and graphene-based materials) in TFN membranes demonstrated non-uniform water-salt permselectivity performance changes with nanoporous fillers. Second, a systematic study was performed investigating different sizes of non-porous (pore-closed) and nanoporous (pore-opened) zeolite particles incorporated into conventionally polymerized TFN membranes; however, the challenges of particle aggregation, non-uniform particle dispersion, and possible particle leaching from the membranes limit analysis. Third, to limit aggregation and improve dispersion on the membrane, a TFN-model membrane synthesis recipe was developed that immobilized the nanoparticles onto the support membranes surface before the polymerization reaction. Fourth, to quantify the possible water transport pathways in these membranes, two different resistance models were employed.

The experimental results show that both TFN and TFN-model membranes with pore-opened particles have higher water permeance compared to those with pore-closed particles. Further analysis using the resistance in parallel and hybrid models yields that water permeability through the zeolite pores is smaller than that of the particle/polymer interface and higher than the water permeability of the pure polymer. Thus, nanoporous particles increase water permeability in TFN membranes primarily through increased water transport at particle/polymer interface. Because solute rejection is not significantly altered in our TFN and TFN-model systems, the results reveal that local changes in the polymer region at the polymer/particle interface yield high water permeability.

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Date Created
  • 2018